dimmer for parallel or series LEDs with timer

[RaZor]

does anyone know where i can get the scheme of dimmer for parallel or series LEDs with timer ?

my LEDs have to be attached to be this way already and I want to connect to a dimmer with a timer

thanks

 

[CDRIVE]

They can't be wired like that. You can't wire LEDs directly in parallel.

Chris

 

[(*steve*)]

They can't be wired like that. You can't wire LEDs directly in parallel.

This is true.

However, there is a caveat. If the LEDs are thermally bonded to the same heatsink, the variation in current between the LEDs may be low enough to prevent thermal runaway (at least at moderate currents).

However the question also asks about a dimmer and a timer. I am still totally unclear as to what interaction is required between them (are the LEDs supposed to (say) slowly dim and go out after a certain period? Or is the dimmer independent of the timer.

Given that the LED current is going to be quite high in this case, I would recommend using PWM to vary the LED brightness.

Something like this might be appropriate.

Note that although a 555 can operate down to 4.5V, 5V doesn't give it a lot of headroom.

 

[KrisBlueNZ]

In addition to what Steve has told you and asked you:

The symbol you're using for your LEDs is wrong.

Do you know the total current consumption of your LED circuit? I expect it's more than 1A.

What behaviour do you want? "A dimmer and timer" is not detailed enough for anyone else to know what you want to do.

 

[RaZor]

Sorry for insufficient information
here more info abaut it :

This is led that I use,
spec :
Forward voltage: 3.2 - 3.8V
Forward current: 350mA
I use resistor 1 Ohm 10 watt.

its get 3.5 volt for every each led.

Now its Running this that way but I want dimmer with timer for it

This is true.

However, there is a caveat. If the LEDs are thermally bonded to the same heatsink, the variation in current between the LEDs may be low enough to prevent thermal runaway (at least at moderate currents).

However the question also asks about a dimmer and a timer. I am still totally unclear as to what interaction is required between them (are the LEDs supposed to (say) slowly dim and go out after a certain period? Or is the dimmer independent of the timer.

Given that the LED current is going to be quite high in this case, I would recommend using PWM to vary the LED brightness.

Something like this might be appropriate.

Note that although a 555 can operate down to 4.5V, 5V doesn't give it a lot of headroom.

The link that gave me, yes something like that but I dont know if that can hold 12 leds running ?

In addition to what Steve has told you and asked you:

The symbol you're using for your LEDs is wrong.

Do you know the total current consumption of your LED circuit? I expect it's more than 1A.

What behaviour do you want? "A dimmer and timer" is not detailed enough for anyone else to know what you want to do.

Yes the simbol is wrong sorry for that, I was in hurry drawing and also my knowledge abaut electroics scheme. I am still beginner

Like what I write above that I want something like the light can goes down and goes up with timer.

 

[KrisBlueNZ]

That's useful information.

I want something like the light can goes down and goes up with timer.

You need to explain what you want MUCH MORE CLEARLY than that.

How do you want to start the timer?
What do you want the LED to do when you've started the timer?

You need to describe what you want CLEARLY and FULLY.

 

[(*steve*)]

Since you're building the circuit yourself, I would STRONGLY recommend you place a resistor in series with each LED.

These are high power LEDs and using a resistor is not really recommended as a current limiting device, however it is better than placing all the LEDs in parallel.

With the appropriate transistor the circuit I gave you can control almost as many LEDs as you can imagine. For larger numbers/higher currents you could use a logic level mosfet.

 

[(*steve*)]

its get 3.5 volt for every each led.

Yes, but you can't be sure that the current is the same and that it stays that way.

 

[KrisBlueNZ]

I totally agree with Steve. Connecting LEDs in parallel is SOMETIMES workable but not likely in this situation. Small differences between the LEDs will mean that some LEDs draw more current than others; they will get hotter, and this will reduce their forward voltage, so they will get hotter, and so on. This is called thermal runaway. Eventually you will have a situation where one LED (or perhaps a few LEDs) draw almost all of the current, and this LED will fail. Hopefully it will fail short, and protect the other LEDs.

Each LED should have its own current limiting resistor. This is my very strong recommendation. If you think it's not important, then think again. You are a beginner here; you've said so. We are not making up rules just to make your job harder. This is a serious issue.

You want to run each LED at 350 mA. These LEDs have a typical forward voltage of 3.5V and you're running them from a 5V supply. That leaves about 1.5V across each series resistor. So the calculation for the series resistor is R = V / I which is 1.5 / 0.35 which is 4.3 ohms. Power dissipation in each resistor will be 525 mW so you should use resistors rated for 1W and make sure they have some form of heatsinking.

As for driving them, the obvious choice would be an N-channel MOSFET with a logic-level gate, with a low ON-resistance. Have a look at http://www.digikey.com/product-detail/en/NTD4906N-35G/NTD4906N-35GOS-ND/2194521. It may seem like overkill, being rated for 10A maximum current, but it has a very low Rds(on) (8 milliohms at Vgs=4.5V) and is among the cheapest suitable parts I could find - cheaper than many other parts with poorer specifications.

This device will switch your LED array ON and OFF under control of its gate, which is driven with a 0V/5V signal from the timer/dimmer circuitry. This signal can be pulse-width modulated (see http://en.wikipedia.org/wiki/Pulse-width_modulation) to vary the LED brightness. You will need a good power supply to be able to supply all those LEDs. Also, avoid long wires, as PWM at high current will cause radiated EMI (interference).

Edit: This MOSFET is excellent for rapid switching of high-current loads with very low ON-resistance but it MUST be driven by a high-current driver so the transition through its linear region is as quick as possible. It is only rated to dissipate 2.6 watts so it must be switched extremely quickly between fully OFF and fully ON, and vice versa.

 

[RaZor]

I'm making a lamp for aquarium I want the led light is just like night and day running 24 hours.
now you can imagine that. simple but for me it is not

Ok I wil change it to series, I use parallel cause I have 5 volt from my PSU 10A.
but if necessary I'll change it to 12volt.

Is The link that @steve gave me is like what I want? can set it up running just like night and day too?
"what is logic level mosfet" ?
how to add fuse to avoid overload voltage?

If the link is like what I want, I would like to try it but might be you have some advice for me before I build it ? I need more simple scheme for begginer like me and also easy to find the type of electronic components that I will buy later.

 

[(*steve*)]

you don't need to change the LEDs to all be in series. (but see Kris' reasons below why it may be better)

What you need to do is to connect a 4.7 ohm 1W resistor to EACH LED (let's say to the anodes).

Then you connect the free end of each of these resistors together.

You then connect all the cathodes together.

The resistor end goes to +ve, the cathodes to -ve

the LEDs all need to be placed on a heatsink. Ensure that the back of the LED isn't electrically connected to the anode.

Get that working first.

Do you have a suitable 5V 10A (ish) power supply?

 

[KrisBlueNZ]

It's not NECESSARY to change to a 12V supply for your LEDs but it will make things easier. You can connect your LEDs in series strings of three. Each string will have a total voltage drop of 10.5V and a current of 350 mA so with a 12V supply, each string will need a series resistor of 4.3 ohms, rated at 1 watt.

The advantage is that your array of 12 LEDs will only draw 1.4A (four strings of three LEDs each, at 350 mA per string) instead of 4.2A (twelve individual LEDs drawing 350 mA each).

It's a lot easier to deal with a 1.4A load, compared with a 4.2A load.

A logic-level MOSFET is a MOSFET that can be driven with a relatively low control voltage from a logic circuit. I linked to one in my previous post.

Now we know that you're making a sunlight simulation for an aquarium, we can advise you on how to control the LEDs.

If I was making this project myself, I would use a microcontroller that would keep track of the time of day and would generate a PWM signal (see the link in my previous post) to control the LED brightness. But I assume you don't have any experience with microcontrollers?

I will think about your requirements and try to draw up a design for you.

 

[CocaCola]

Personally if it was me I would just get a constant current driver, like this... If you want a simple on/off you can then just use a simple on/off light timer, like you use for say an Xmas tree... If you want to dim and fade, that particular driver will also allow dimming but it will require a little more work... If you don't need dimming there are cheaper drivers to be had, like this that should run your 12 x 1W(ish) LEDs just as well...

BTW, make sure to heat sink those LEDs well or they won't last long...

 

[KrisBlueNZ]

I tried to think of a way to design your 24-hour brightness controller (to simulate sunlight over each 24-hour period) without an MCU (microcontroller) but it is just not workable without some kind of programmability.

Do you have any programming experience? The hardware requirements are not great; perhaps you have an old laptop with a parallel port that could control a PWM circuit?

Otherwise, the proper way to do this would be with a microcontroller such as an AVR, PIC, 8051 etc. It would allow you to set the time of day, and it would keep track of the time of day, and use a table to control an analogue or digital output that would control a PWM circuit that would drive the LEDs.

There are several people on this forum who could do this for you, for a fee. BobK would probably do it, and CocaCola as well. If you're prepared to pay, you could send them a private message. Otherwise, you need to find someone who can do the programming for you, or learn programming yourself.

 

[CocaCola]

Kris not to dumb it down (I know that is not your style) but with 12 LEDs you could get a pretty decent fade in and out by just stepping across the 12 LEDs one at a time...

Yeah going 1 led on, 2 leds on, 3 leds on is hardly smooth but overall it's something that can be done a lot easier without the need for programming... Although there is still some issues with timing drift and lack of an easy way to adjust for seasons and what not but just the same something kinda simple could be worked up...

Heck if you really want to get all full blown (still using a step logic) you could always manually stepping across fixed resistor banks as well to get some different fades... Combine the two and I believe a pretty smooth effect could be achieved, even if it wasn't perfectly smooth or efficient...

Something like

LED 1 50%
LED 2 50%
LED 1 100%
LED 3 50%
LED 2 100%

Or something along those lines

Granted I can see the circuit getting big real quick and kinda overkill compared to doing it with a micro, but well, I guess that's the price you pay trying to avoid using a micro

There are of course over the counter solutions, like dusk to dawn green house lighting timers with dimming (that would work with the dimmable LED driver I linked) but they can get costly real quick from my fast Google search...

 

[KrisBlueNZ]

The need for programming that I see is the need to keep track of the time of day and translate it to an intensity figure that corresponds to the average daylight intensity at that time of day.

It doesn't need to be highly accurate, but I would expect it would need to ramp up during the morning and down at night. And for convenience you would need to be able to enter the time of day at any time. It might not need to have a non-volatile clock.

If there's an over-the-counter solution for greenhouse lighting control, that might be usable. I assume the dimming would be done using an SCR or triac. In that case, it might be possible to hack it into an LED driver circuit.

 

[CocaCola]

I assume the dimming would be done using an SCR or triac. In that case, it might be possible to hack it into an LED driver circuit.

There are two types of dimmable LED drivers, some that require the kinda industry standard dimmer signal (these would need the appropriate circuitry to dim) and those like the one I linked that will work with normal over the counter lighting dimmers... So hacking it's really needed just plugging it together...

I understand what you mean in regards to setting for average daylight cycles, that gets overly complicated to automate I was figuring a more static on/off cycle if you get away from micros...

But, at the end of the day once you get away from a DIY solution and having to pay someone to build this, you can pretty much afford one of the over the counter greenhouse or reef system controllers that are already designed to do just this... Just Google 'sunrise sunset simulator' or something like that and read the hits, most of the hits are for animal breeders or fish lighting forums (some green house ones) and they link to over the counter options...

 

[RaZor]

Yes I only have 5volt 250Watt with large ampere 15-20A (I am not sure) but between that values.
yes there is availiable heatsink like this :

but I made DIY again with large heatsink on them.

Here my project I made already.

Each group has one switch on/off (still manualy)
Each group has different transistor values.
All led in parallel but only red led has no resistor cause handle 2.5volt for max current.
others 3.5v.

I would like to use a timer for each group, so its need 6 timer.
I have timer for AC but here DC timer cause for led that its need and I want to make it DIY if there is avaliable.

Now cause I dont want make shock for my fish (because its too bright) so I would like to make it dimmer for leds light. (maybe each group)

for easier installation timer and dimmer circuit should made from different cicuits,
thats what I though.. probably there is scheme (timer and dimmer) that I can start to build.

No no I dont know microcontroler or something like that
I know visual delphi program little bit but I never use it for operating device.

 

[RaZor]

With the appropriate transistor the circuit I gave you can control almost as many LEDs as you can imagine. For larger numbers/higher currents you could use a logic level mosfet.

I would like to try what you gave that link but what the appropriate transistor that can work for my DIY ?
seem its simple circuit I need more advice.

 

[(*steve*)]

See this thread. I just gave someone some advice which is pretty much exactly what you need.